1. Field of the Invention
The present invention relates generally to mixing enhancement between two or more fluids, at least one of the fluids being a gas supplied under pressure. More particularly, it relates to mixing enhancement in fuel injectors, sprays, chemical lasers, inside the exhaust duct of turbine engines, outside the exhaust duct of jet engines, and other related applications.
2. Description of Related Art
Mixing enhancement is desirable in a large variety of processes and devices, including combustion, propulsion, fluid pumping, chemical lasers, materials processing, and spray coating.
In every combustion system, fuel and oxidizer must be mixed thoroughly and in the correct proportion before reaction takes place. Otherwise, combustion is incomplete, leading to reduced efficiency and excessive production of pollutants. Complete mixing is often difficult because of the limited space and/or time available for the reactants to mix thoroughly. This difficulty is augmented in cases when the velocity of the flow inside the combustion chamber is high subsonic (e.g., Mach number M≅0.8), sonic (M=1), or supersonic (M greater than 1), because mixing is suppressed with increasing Mach number.
To overcome the above difficulty in large-scale combustion systems, prior art mixing enhancement devices such as swirlers, ramps, and lobe mixers are incorporated in injectors for fuel and/or oxidizer. However, these mechanical mixers decrease the momentum of the flow, thus compromise the penetration of the reactants into the combustion zone. Penetration, which is proportional to the fluid momentum, is crucial in many combustion schemes. Because mechanical mixers increase the fluid resistance of the injector, higher pumping power must be used to deliver the same amount of reactant. Furthermore, mechanical mixers cause total pressure losses which in turn cause decreased system efficiency and, in propulsion devices, loss of thrust.
In small-scale combustion applications, such as in a piston cylinder of a diesel engine, mechanical mixers would be very costly to install and maintain because of their complex shapes.
Therefore, there is a current need for a simple and efficient mixing enhancement scheme which is easy to implement and maintain for use in both large-scale and small-scale combustion applications.
In jet propulsion systems, it is often required to reduce jet noise by enhancing fluid mixing between the jet exhaust and the ambient air or, in the case of certain turbofan engines, by enhancing mixing between the core stream and the fan stream inside the exhaust duct. The same mixing also reduces the thermal signature of the jet exhaust. The typical device used for mixing inside or at the exit of the exhaust of the engine is the lobe mixer. Although lobe mixers can provide adequate mixing, they reduce thrust and increase the weight and complexity of the engine. The thrust reduction is particularly severe when the engine exhaust is supersonic.
Therefore, there is a current need for a simple, lightweight and efficient mixing enhancement scheme which is easy to implement and maintain for use in jet propulsion systems.
Performance of ejectors depends on the rapidity of mixing between the motive fluid and the fluid entrained into the ejector. In aircraft engines, where ejectors are often used for noise reduction, mixing enhancement between the engine exhaust (which acts as the motive fluid) and the ambient air entrained into the ejector is accomplished typically via lobe mixers. As discussed above, lobe mixers penalize engine performance.
In industrial fluid pumping applications, ejectors rarely employ mechanical mixers, relying instead on the natural mixing between motive and entrained fluids. Mechanical mixers are avoided because they would increase the complexity, manufacturing costs, and maintenance expenses of the pump. Thus, there is a current need for an effective mixing enhancement scheme which is geometrically simple and easy to implement for use in ejector pumps.
Therefore, there is a current need for a simple and efficient mixing enhancement scheme which is easy to implement and maintain for use in ejectors.
The efficiency of chemical lasers depends on the completeness of mixing between the reactant gases used for lasing. Each of the reactant gases enters the laser cavity via an injector. Since the gases enter at supersonic speeds, mixing is very slow. Enhancing the mixing of gases in this case with installation of a mechanical mixer on each injector is impractical due to the large number of injectors.
Therefore, there is a current need for an effective mixing enhancement scheme having simple geometric shapes to improve performance of chemical lasers.
In several materials processing schemes, a molten metal is atomized into spray droplets by the action of a pressurized gaseous jet, then deposited on a surface according to a specified pattern. Mixing enhancement of the gaseous jet facilitates the atomization process. Mixing enhancement is also needed in a variety of spray coating applications, where fine atomization of the coating liquid enables homogeneous deposition. Because the dimensions of jet nozzles used in spray depositions are very small, installation of mechanical mixers is impractical.
Therefore, there is a current need for a simple and efficient mixing enhancement scheme which is easy to implement and maintain for use in spray deposition devices.
The present invention is a method and an apparatus for enhancing fluid mixing. The method comprises the following: (a) configuring a duct to have an effective outer wall, an effective inner wall, a cross-sectional shape, a first cross-sectional area and an exit area, the first cross-sectional area and the exit area being different in size; (b) generating a first flow at the first cross-sectional area, the first flow having a total pressure and a speed equal to or greater than a local speed of sound; and (c) generating a positive streamwise pressure gradient in a second flow in proximity of the exit area. The second flow results from the first flow. Fluid mixing is enhanced downstream from the duct exit area.